Method for intercepting control data, in particular quality of service data, and associated device

ABSTRACT

A device comprises interception means ( 2 ) capable, in the case of transfer of data packets between at least two remote terminals (Tij-k), via a communications network, of intercepting at least certain of the control packets during the transfer, and of determining amongst the intercepted control packets those which are formatted according to a chosen real-time data transfer control protocol, and management means ( 3 ) capable of duplicating at least part of each intercepted control packet, and of generating data, representing the duplicated part, intended to be communicated to control means ( 1 ) located in a control application (S) of the network.

FIELD OF THE INVENTION

The invention relates to the field of communications between terminalswithin a network, and more particularly that of control of the dataexchanged between terminals.

BACKGROUND OF THE INVENTION

Certain operators or access providers have developed methods (orapplications) allowing their clients to exchange data packets within acommunications network under preferential conditions (or with a certainquality of service level). In order to allow these clients to check (orcontrol) the actual transfer conditions of data of “stream” type, inparticular dedicated to video and voice, and transported, for example,according to the RTP protocol (Real-time Transfer Protocol), the RTCPprotocol (Real-time Transfer Control Protocol) is used. This protocol,developed for controlling the real-time transfer of data (especiallyaudio and/or video), allows in particular the sender of a stream toreceive, in real time, information characterising the data transfer,such as the percentage of data packets lost, or the variation in packettransmission time. The RTP protocol encapsulates the data, numbers andtimestamps the packets, and RTCP packets are sent back by the receiverof the RTP stream to the sender, in order to communicate theretoinformation on the transfer, principally the number of RTP packets lost.

Users, who have possibly paid in order to have preferential conditions,can thus, when they are not satisfied, question their operator (or theirservice provider) in order to obtain explanations, or discounts.However, in the case of micro-streams (or data exchanges between two endusers), such as for example conventional video sessions on the Internet,operators find it difficult to access (in real time or off-line)statistics relating to the quality of the communication, so that theycannot know that a quality problem has appeared, and therefore cannotreact in an appropriate manner (possibly disproving the complainant).

The aim of the invention is therefore to remedy this drawback.

SUMMARY OF THE INVENTION

To that end it proposes a method for intercepting control data exchangedby remote terminals, via a communications network, in the form ofcontrol packets formatted according to a first real-time data transfercontrol protocol (such as for example RTCP) and associated with datapreviously exchanged by these terminals (generally in the form ofpackets formatted according to a second real-time data transfer protocol(such as for example RTP)). The invention relates to the interception ofat least some of the control packets, which are formatted according tothe first protocol and which are in the process of being transferred,with a view to communication, immediate or delayed, to a controlapplication located in the network, after total or partial duplication,of data representing the duplicated parts, so that the controlapplication deduces therefrom information on the transfer (in particularthe quality of service in the case of the RTCP protocol).

“Duplication” means here the fact of retrieving data in order totransmit them to an application, but also the fact of storing these datain so-called log files with a view to off-line processing.

By virtue of this device, based on duplication with possible selectionof data, operators (or access providers) can have available in real time(or off-line) the same transfer information as their clients.

The method according to the invention can comprise many additionalcharacteristics which can be taken separately and/or in combination, andin particular:

-   -   interception of all the control (for example RTCP) packets        transferred, or of one packet out of n (n being a chosen integer        value);    -   determination of the packets in which at least the network        address field for the terminal which sent the packet, the        network address field for the destination terminal of the        packet, the destination port field and/or the source port field,        and the protocol (such as for example UDP) number field, have        chosen values (or filters). These chosen values (or filters) are        preferentially transmitted by an application (possibly the        destination control application for the duplicated data) and/or        from another item of equipment in the network;    -   between interception and duplication, comparison between a        chosen threshold value and the value of a service information        field contained in the intercepted control packet (comprising,        preferably, data representing the quality of service), in order        to intercept/duplicate only control packets in which the service        information field has a value substantially greater than the        threshold value (notion of filtering). In this case, the whole        of each intercepted control packet (therefore formatted        according to the first protocol) and having a service        information field with a value substantially greater than the        threshold value may be duplicated, in order to communicate the        whole of the duplicated control packet. In a variant, only        certain chosen fields contained in each intercepted control        packet (therefore formatted according to the first protocol) and        having a service information field with a value substantially        greater than the threshold value may be duplicated, in order to        communicate only the duplicated fields. Preferentially, in this        variant, the service information field or fields are also        duplicated, in order to communicate it or them with the other        duplicated fields. In a variant, instead of communicating the        service information field or fields, information data which        represent it or them are communicated;    -   duplication (without filtering) of the whole of each intercepted        control packet (therefore formatted according to the first        protocol) or, in a variant, of certain chosen fields contained        in each intercepted control packet (therefore formatted        according to the first protocol), including at least the service        information field or fields. In a variant, instead of        communicating the service information field or fields,        information data which represent it or them are communicated;    -   preferential duplication of the detected fields of the network        address for the terminal which sent the packet, the network        address for the destination terminal of the packet, the        destination port and/or the source port, and the protocol        number.

The invention also relates to a device for intercepting control dataexchanged by remote terminals, via a communications network, in the formof control packets formatted according to a first real-time datatransfer control protocol (such as for example RTCP) and associated withdata previously exchanged by these terminals (generally in the form ofpackets formatted according to a second real-time data transfer protocol(such as for example RTP)).

More precisely, the device is characterised by the fact that itcomprises, on the one hand, interception means capable, in the case oftransfer of control data packets between at least two remote terminals,of intercepting those which are formatted according to the firstprotocol, and, on the other hand, management means capable ofduplicating at least part of each intercepted control packet, and ofgenerating data representing the duplicated part, in order that they arecommunicated (immediately or delayed) to control means located in acontrol application of the network.

The device according to the invention can comprise many additionalcharacteristics which can be taken separately and/or in combination, andin particular:

-   -   interception means organised for intercepting all the control        packets transferred with a view to determining their format, or        else only one packet out of n (n being a chosen integer value),        after having sampled (or filtered) the control packets;    -   interception means organised, on the one hand, for detecting        from amongst all the packets those in which at least the address        fields designating the terminal which sent the packet, the        destination terminal of the packet, the destination port and/or        the source port, and the protocol number have chosen values,        and, on the other hand, for retaining the packets having these        chosen values (or filters), these then being referred to as        “intercepted control packets”. These chosen values (or filters)        are preferentially transmitted to the device by an application        (possibly the destination control application for the duplicated        data) or from another item of equipment in the network;    -   interception means organised for detecting at least a service        information field contained in each intercepted control packet,        and for performing, between interception and duplication, a        comparison between a stored chosen threshold value and the value        of the detected service information field (comprising data        representing the quality of service in the case of the RTCP        protocol, in particular), so that the management means duplicate        only the part at least of the control packet in which the        service information field has a value substantially greater than        the threshold value. In this case, the interception means are        preferentially organised for communicating to the management        means the whole of each intercepted control packet having a        service information field with a value substantially greater        than the threshold value, and the management means are        preferentially organised for duplicating the whole of each        intercepted control packet received and communicating to the        control means the whole of the duplicated control packet. In a        variant, the interception means are organised for communicating        to the management means certain chosen fields contained in each        intercepted control packet having a service information field        with a value substantially greater than the threshold value, and        the management means are organised for duplicating the chosen        fields of each intercepted control packet received and        communicating the duplicated fields to the control means.        Preferentially, in this variant, the management means also        duplicate the service information field or fields, in order to        communicate it or them with the other duplicated fields. In a        variant, instead of communicating the service information field        or fields, the management means communicate information data        which represent it or them;    -   management means organised for duplicating certain chosen fields        contained in each intercepted control packet (therefore        formatted according to the first protocol), including at least a        service information field. Preferentially, the management means        are organised for communicating, with the other duplicated        fields, information data representing the duplicated service        information field;    -   management means organised for duplicating the detected fields        of the network address for the terminal which sent the        intercepted control packet, the network address for the        destination terminal of the intercepted packet, the destination        port and the protocol number, and for communicating these        duplicated fields to the control means;    -   management means organised for duplicating the whole of each        intercepted packet (therefore formatted according to the first        protocol), and for communicating to the control means the whole        of the duplicated control packet;    -   interception means and/or management means located in at least        one of the items of network equipment through which the streams        intended for the terminals flow. The equipment can be, for        example, a firewall, a router, a NAT box (RFC 2663 and RFC 3022)        or a traffic manager such as a traffic shaper.

The invention can be used in any type of communications network, privateor public, and in particular in Internet networks. Furthermore, thefirst and second protocols according to which the data packets areformatted are preferentially the RTCP and RTP protocols, respectively.Moreover, the duplicated data can be communicated according to the COPSor SNMP protocol or, better still, according to an encapsulationprotocol taking the retained RTCP packets and integrating them as theyare into other packets, sent directly, immediately or delayed, to theverification application.

Other characteristics and advantages of the invention will emerge froman examination of the following detailed description, and of theaccompanying drawing in which the single FIGURE illustratesschematically a communications installation equipped with a deviceaccording to the invention. This drawing can serve, not only to add tothe invention, but also to contribute towards its definition, if needbe.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE schematically illustrates a communicationsinstallation equipped with a device according to an exemplary embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

The communications installation illustrated in the single FIGUREcomprises first of all a server S connected to a communications network,partially embodied by two-way arrows. As a non-limiting example, it isconsidered in the following description that the network is the publicInternet network in which the data are exchanged according to the IPprotocol.

As illustrated, in this example the network comprises central routersRC-k connected to the server S and to a multiplicity of edge routersRPj-k. User terminals Tij-k are connected to the various edge routersRPj-k. These terminals are either fixed or mobile. They can be, forexample, fixed or portable computers, fixed or portable telephones, orpersonal digital assistants (PDAs).

Furthermore, it is considered that the terminals can exchange in realtime, with other terminals, audio and/or video over IP (VoIP) sessionsor multimedia over IP (MMoIP) sessions. It is also considered that theseterminals use the RTP protocol (Real-time Transfer Protocol) and RTCPprotocol (Real-time Transfer Control Protocol) for exchanging themultimedia streams.

As these protocols are well known to persons skilled in the art, theywill not be presented in detail. Their main functions andcharacteristics are simply noted here.

The RTP protocol makes it possible to provide a uniform means oftransmission over IP of data subject to constraints of real-timetransfer, either “point to point” (or “unicast”; a micro-stream betweentwo terminals), or “multipoint” (or “multicast”; a stream from oneterminal to several terminals). It uses IP packet sequence numbers whichmake it possible to reconstruct audio and/or video information,including when the network changes the order of the packets. RTP thusmakes it possible to add time markers or sequence numbers to the datapackets, to identify the type of information transported (uniquesynchronisation source identifier (SSRC)) and to control the arrival ofthe packets at their destination.

RTP is a protocol using the UDP (User Datagram Protocol) underlyingtransport protocol. The RTCP protocol makes it possible to control theRTP streams. It is based on periodic transmissions of control packets bythe different participants in a session. It therefore makes it possibleto convey information on the participants and on the quality of service(QoS). More precisely, it makes it possible to provide feedback for asource (terminal); in addition it makes it possible to reveal individualor group distribution faults; it also makes it possible to keep a traceof the different participants (by virtue of a unique and permanentidentifier for each participant (CNAME) and a synchronisation sourceidentifier (SSRC)); it also makes it possible to control the rate atwhich the participants in an RTP session transmit their RTCP packets;finally it makes it possible to transmit control information on thesession (for example in order to identify a participant on the screensof the other participants).

RTP and RTCP use separate ports of a pair of ports (usually the evenport for RTP and the odd port immediately above for RTCP).

As explained in the introduction, the invention relates to theinterception of at least part of the RTCP control packet stream in theprocess of being transferred within the network. In the followingdescription, “RTCP control packet” will mean a control data packetformatted according to the (first) RTCP protocol. Similarly, “RTPpacket” will mean a data packet formatted according to the (second) RTPprotocol.

It is a question in effect of detecting all the RTCP control packets, oronly some of them, in order to communicate them, after total or partialduplication, either substantially in the same form or in the form ofdata which represent them, to a control application 1 located in thenetwork, for example in the server S of the operator or the accessprovider of the users. This control application 1 can be a call controlserver (such as “SIP proxy” or “H.323 gatekeeper”), or any other type ofequivalent controller, such as for example a passband controller,whether in hardware or software form. The control application 1 can thendeduce therefrom information on the transfer of the data packets, suchas for example the quality of service (QoS), defined by one or moreservice information data fields (higher number of packets received,number of packets lost, transfer time between terminals, jitter betweensuccessive arrivals).

In order to achieve this objective, there is provided, first of all, atleast one interception module 2 (hereinafter referred to as a filter)located in at least one of the routers in the network. As interceptionof the RTCP control packets is expensive to implement (in particularowing to the large number of micro-stream filterings in a core router),it is preferable to perform it in items of equipment, such as therouters, close to the terminals.

Preferentially, the filter 2 intercepts all control packets liable to beformatted according to the RTCP protocol. But, in a variant, it canperform a sampling (or filtering) of the control packets, so as tointercept only one control packet out of n (n being a chosen integervalue, for example equal to 2 or 3).

As the object of the interception is to “retain” only RTCP controlpackets, the filter 2 must consequently analyse the packets in theprocess of being transferred. This analysis concerns preferentiallychecking the values of at least four (perhaps even five) fields: theprotocol number field in the IP header (the value must be UDP), thenetwork address field for the terminal which sent the packet, thenetwork address field for the destination terminal of the packet, andthe destination port field and/or the source port field. A packet whichhas at least four (perhaps even five) chosen field values is a candidatefor interception.

These chosen values (also referred to as filters) are preferentiallytransmitted to the device (and in particular to its filter 2) by anapplication, which is possibly the control application 1 which is thedestination for the duplicated data, or by another item of equipment inthe network. These filters (or chosen values) can be transmittedaccording to a protocol such as COPS.

The device according to the invention also comprises at least onemanagement module 3 located in an item of network equipment, for examplethe one which comprises the interception module 2, with which it thenconstitutes the filter (as illustrated). In the following description,filter will designate both the interception module 2 and the managementmodule 3.

Each management module 3 is designed to duplicate at least part of eachcontrol packet intercepted and then communicated by the interceptionmodule 2 with which it is associated, and to generate data representingthe duplicated part, in order that they are communicated to the controlapplication 1. Communication of the duplicated part can be immediate ordelayed. In the delayed case, the duplicated parts are stored in logfiles before being communicated to the control application 1.

Between the interception and duplication steps, a filtering step can beprovided, consisting in communicating, with a view to duplication, onlythe RTCP control packets comprising a service information field with avalue greater than a threshold value. This is because it can beconsidered that, below this threshold value, the quality of service isacceptable and therefore it is not necessary to feed back theinformation to the control application 1. For example, the thresholdvalue relating to the percentage of lost packets is chosen equal to 1%,so that any RTCP control packet having a loss greater than 1% forms thesubject of a “report” by duplication.

It is quite obvious that this notion of threshold is relative. Thecomparison can in fact be carried out on the value of the percentage ofpackets transmitted. In this case, there would be communicated, with aview to duplication, only the RTCP control packets comprising a serviceinformation field with a value less than a threshold value, for example99%.

Furthermore, the filtering can concern the values of several serviceinformation fields. In this case, a threshold value is provided for eachfield having to be subject to a comparison.

In order to perform the filtering, the interception module 2 istherefore organised so as to detect the service information field, theobject of the comparison, contained in the intercepted packets, and thento extract its value in order to compare it with the stored chosenthreshold value. Once the comparison has been performed, the processingof the packet continues normally in the router (or network equipment)and a copy of at least part of the packet is communicated to themanagement module 3 depending on whether the value of its serviceinformation field is less than or greater than the threshold.

Preferentially, the management module 3 duplicates everything it hasreceived from the interception module 2, whether this is the whole of acontrol packet or only parts thereof. But in a variant it can beenvisaged that the management module 3 is organised so as to duplicateonly some of the data it has received. This situation can in particularbe envisaged when the interception module 2 communicates to it the wholeof the intercepted control packet. On the other hand, this solution isnot of real interest when the interception module transmits only part ofthe intercepted control packet, for example the network address fieldfor the terminal which sent the packet, the network address field forthe destination terminal of the packet, the destination port field, theprotocol number field, and the service information field or fieldscontained in the packet.

It is important to note that the management module 3 can be organised soas to communicate information data which represent the serviceinformation field or fields, rather than the contents of these fields.For example, a field can be replaced by an alarm or a bit whose 0 or 1value indicates that an RTP packet has been lost or transmitted.

Transmission (or communication) of the duplicated elements, originatingfrom the RTCP control packets, between the management module 3 and thecontrol application 1 can be performed by any appropriate means. Use canbe made for example of the COPS (Common Open Policy Service—RFC 2748)protocol coupled with a packet duplication request PIB (PolicyInformation Base), or the SNMP (Simple Network Management Protocol—RFC1157) network management and administration protocol coupled with an MIB(Management Information Base), or the CMIS/CMIP protocol also coupledwith an MIB.

The interception, management and control modules can be respectivelyimplemented in the form of electronic circuits, software (or computer)modules, or a combination of circuits and software. Furthermore, themanagement and interception modules can be grouped together in one andthe same module forming a filter.

Furthermore, the device can be configurable as a function of the users.In fact, a number of different threshold levels associated withdifferent categories of service can be envisaged.

Moreover, the modules presented above, which constitute the deviceaccording to the invention, can be located in any type of equipment inthe network given that this equipment is installed on the path of thecontrol packets formatted according to the first protocol (here RTCP).Amongst these items of equipment, there can in particular be citedrouters (edge or core), firewalls, NAT boxes (Network AddressTranslation boxes) or Traffic Shapers (traffic managers).

In addition, it is possible to use the control application, for examplelocated in the server S, for transmitting filters (for example the four(perhaps even five) chosen values of the aforementioned fields and/orthreshold values) to the items of equipment in which the modulesconstituting the device according to the invention are located.

The invention also offers a method for intercepting the control dataexchanged by remote terminals, via a communications network, in the formof control packets formatted according to a first real-time datatransfer control protocol (such as for example RTCP) and associated withdata previously exchanged by these terminals (generally in the form ofpackets formatted according to a second real-time data transfer protocol(such as for example RTP)).

This can be implemented by means of the device presented above. As themain and optional functions and sub-functions provided by the steps ofthis method are substantially identical to those provided by the variousmeans constituting the device, only the steps implementing the mainfunction of the method according to the invention will be summarisedbelow.

This method comprises a step in which i) at least certain of the controldata packets which are in the process of being transferred on thenetwork between at least two remote terminals are intercepted, so as todetermine those which are formatted according to the first protocol(here RTCP), then ii) at least part of each control packet thusformatted is duplicated, and iii) data representing the duplicated partare communicated to a control application located in the network, sothat it deduces therefrom information on the transfer (and principallythe quality of service in the case of the RTCP protocol).

The invention is not limited to the embodiments of the methods anddevices described above, solely by way of examples, but it includes allthe variants that can be envisaged by persons skilled in the art withinthe context of the following claims.

1. A method for intercepting data exchanged by remote terminals via a communications network, said data including control packets formatted according to a first real-time data transfer control protocol and associated with data previously exchanged by said terminals, the method comprising: intercepting at least certain data packets of a transfer between at least two remote terminals, during said transfer, so as to determine which of said data packets are control packets, said control packets being formatted according to said first protocol; duplicating at least part of each of said control packets; communicating data representing said duplicated part of each of said control packets to a control application located in said network, said control application deducing information on said transfer from said communicated data; between said intercepting and said duplicating, performing a comparison between a chosen threshold value and the value of a service information field contained in the intercepted control packet; wherein said duplicating comprises duplicating only the part of the control packet in which the service information field has a value greater than said chosen threshold value; wherein certain chosen fields contained in each intercepted control packet, formatted according to the first protocol and in which the service information field has a value greater than said chosen threshold value, are duplicated, and said duplicated chosen fields are communicated; and wherein said service information field is also duplicated, and information data representing said duplicated service information field are communicated with said duplicated chosen fields.
 2. The method according to claim 1, wherein all the control packets of said transfer between at least two remote terminals are intercepted.
 3. The method according to claim 1, wherein the control packets are sampled so as to intercept only one sample from amongst n, n being a chosen integer value.
 4. The method according to claim 1, wherein determination of the formatting according to the first protocol concerns the determination, amongst the packets, of those in which at least a network address field for the terminal which sent the packet, a network address field for the destination terminal of the packet, a destination port field and/or a source port field, and a protocol number field have chosen values.
 5. The method according to claim 4, wherein said chosen values are communicated by an application and/or an item of equipment in the network.
 6. The method according to claim 1, further comprising: between said intercepting and said duplicating, performing a comparison between a chosen threshold value and the value of a service information field contained in the intercepted control packet; wherein the whole of each intercepted control packet, formatted according to the first protocol and in which the service information field has a value greater than said chosen threshold value, is duplicated, and the whole of said duplicated control packet is communicated.
 7. The method according to claim 1, wherein one of the duplicated and communicated chosen fields is said service information field.
 8. The method according to claim 1, wherein certain chosen fields contained in each intercepted control packet, formatted according to the first protocol, including at least a service information field, are duplicated.
 9. The method according to claim 8, wherein information data representing said duplicated service information field are communicated with the other duplicated fields.
 10. The method according to claim 1, wherein the service information field comprises data representing a quality of service.
 11. The method according to claim 4, wherein: certain chosen fields contained in each intercepted control packet, formatted according to the first protocol and in which the service information field has a value greater than the said threshold value, are duplicated, and said duplicated fields are communicated; and said detected network address field for the terminal which sent the packet, said detected network address field for the destination terminal of the packet, said detected destination port field, and said detected protocol number field are duplicated.
 12. The method according to claim 1, wherein the whole of each intercepted control packet, formatted according to the first protocol, is duplicated.
 13. A device for intercepting data exchanged by remote terminals, via a communications network, said data including packets formatted according to a first real-time data transfer control protocol and associated with data previously exchanged by said terminals, the device comprising: interception means for intercepting at least certain data packets of a transfer between at least two remote terminals, during said transfer, and for determining amongst the intercepted data packets which of said data packets are control packets, said control packets being formatted according to said first protocol; and management means for duplicating at least part of each of said intercepted control packets, and for generating data representing said duplicated part of each of said control packets, intended to be communicated to control means located in a control application of said network; wherein said interception means are organized for detecting a service information field contained in each intercepted control packet, and for performing, between interception and duplication, a comparison between a stored chosen threshold value and the value of the detected service information field, so that the management means duplicate only the part at least of the control packet in which the service information field has a value greater than said threshold value; wherein said interception means are organized for communicating to said management means certain chosen fields contained in each intercepted control packet in which the service information field has a value greater than said threshold value, and said management means are organized for duplicating said chosen fields of each intercepted control packet received and communicating said duplicated fields to said control means; and wherein said interception means are organized for communicating said service information field to said management means, and said management means are organized for duplicating said service information field and communicating, with other duplicated fields, information data representing said duplicated service information field.
 14. The device according to claim 13, wherein said interception means intercept all control packets transferred.
 15. The device according to claim 14, wherein said interception means sample the control packets in the process of being transferred, and intercept only one sample from amongst n, n being a chosen integer value.
 16. The device according to claim 13, wherein said interception means detect from amongst the packets those in which at least a network address field for the terminal which sent the packet, a network address field for the destination terminal of the packet, a destination port field and/or a source port field, and a protocol number field have chosen values; and retain the packets having said chosen values, these packets then being referred to as intercepted control packets.
 17. The device according to claim 16, wherein said interception means are organized for receiving said chosen values from at least one of an application and an item of equipment in the network.
 18. The device according to claim 13, wherein said interception means are organized for communicating to said management means the whole of each intercepted control packet in which the service information field has a value greater than said threshold value, and said management means are organized for duplicating the whole of each intercepted control packet received, and communicating to said control means the whole of said duplicated control packet.
 19. The device according to claim 13, wherein one of the duplicated and communicated fields is said service information field.
 20. The device according to claim 13, wherein said management means are organized for duplicating certain chosen fields contained in each intercepted control packet, formatted according to the first protocol, including at least a service information field.
 21. The device according to claim 13, wherein said management means are organized for communicating information data, representing the said duplicated service information field, in addition to other duplicated fields.
 22. The device according to claim 13, wherein the service information field comprises data representing the quality of service.
 23. The device according to claim 16, wherein said interception means are organized for detecting a service information field contained in each intercepted control packet, and for performing, between interception and duplication, a comparison between a stored chosen threshold value and the value of the detected service information field, so that the management means duplicate only the part at least of the control packet in which the service information field has a value greater than the said threshold value; and said management means are organized for duplicating said network address field for the terminal which sent the intercepted packet, said network address field for the destination terminal of the intercepted control packet, said destination port field and said protocol number field, and for communicating said duplicated fields to the control means.
 24. The device according to claim 13, wherein said management means are organized for duplicating the whole of each intercepted control packet, formatted according to the first protocol, and for communicating to said control means the whole of the said duplicated control packet.
 25. The device according to claim 13, wherein said interception means are located in at least one of the items of network equipment through which the streams intended for the said terminals flow.
 26. The device according to claim 13, wherein said management means are located in at least one of the items of equipment in the network to which the said terminals are connected.
 27. The device according to claim 25, wherein the network equipment is chosen from a group comprising routers, NAT boxes, firewalls and traffic shapers.
 28. The device according to claim 13, wherein said network is at least one of a public and a private network.
 29. The device according to claim 28, wherein said network is the Internet.
 30. The device according to claim 28, wherein the first protocol is called RTCP, and is associated with a real-time data transfer protocol called RTP.
 31. The device according to claim 28, wherein the duplicated data are communicated according to a protocol chosen from a group comprising the COPS and SNMP protocols, and encapsulation protocols. 